The characteristics of spheroidization decline: good spheroidization before furnace, poor spheroidization on castings; or good spheroidization of castings poured first and poor spheroidization of castings poured later in the same ladle.
The reason of spheroidizing decline is that the amount of magnesium and rare earth decreases with the prolongation of stopping time. The affinity of magnesium and rare earth with oxygen is greater than that with sulfur. Therefore, the following reactions should take place between the MGS and ce2s3 inclusions on the surface of the molten iron and the oxygen in the air: 2mgs + O2 = 2MgO + 2S 2ce2s3 + 3O2 = 2ce2o3 + 6S. At this time, the sulfur produced enters the molten iron again and reacts with magnesium and rare earth: Mg + S = MGS
In this way, with the increase of the stop time of the molten iron, the sulfur will continuously react with the magnesium and the rare earth to generate MgS and Ce2S3, which will be continuously oxidized by the oxygen in the air and recycled. As a result, magnesium and rare earth in the molten iron are consumed, and sulfur enters the molten iron again from the scum, thus “sulfur recovery” appears.
The boiling points of rare earth cerium and yttrium are higher than that of magnesium, and they will not vaporize at ordinary molten iron temperature. In addition, the sulfides and oxides of rare earth cerium and yttrium have high melting point, high density and slow floatation. Therefore, the slowing down rate of rare earth cerium and yttrium is smaller than that of magnesium. At 1350 ~ 1400 ℃, the mass fraction of magnesium decays (0.001 ~ 0.004)% / min; the mass fraction of light rare earth cerium decays (0.0006 ~ 0.002)% / min; the mass of heavy rare earth yttrium decays (0.0008% / min). The decay rate of various spheroidizing elements is closely related to the sulfur content in molten iron. The higher the sulfur content, the faster the decay rate.
Measures to reduce the spheroidization recession are listed as follows:
(1) Shorten the stopping time of molten iron: from the completion of spheroidizing treatment to the completion of pouring, it should be finished within 15 minutes.
(2) Reduce the sulfur content of the original liquid iron: if the sulfur content of the original liquid iron is high, more nodulizing elements will be consumed. In addition, if the sulfur content of the original liquid iron is high, the sulfur content in the slag will increase, which will intensify the “sulfur recovery phenomenon” and accelerate the nodulization decline.
(3) Strengthen covering and slagging: after spheroidizing treatment, add rare slagging agent (such as perlite) to cover and take slagging measures for many times, which can reduce “sulfur recovery phenomenon”.
(4) It is feasible and effective, but not the best, to appropriately increase the amount of nodulizer according to the sulfur content in the molten iron and to take corresponding measures to increase the amount of nodulizer. The solution is to minimize the sulfur content in the molten iron. In addition, excessive addition of spheroidizing agent not only increases the cost, but also leads to the deterioration of graphite spheroidization.